1. Field of the Invention
The present invention relates to a system of transmission by modulation-demodulation of a microwave, for two-way communication between a fixed station, called a beacon or reader, and a mobile station, called a badge or responder. More precisely, the invention relates to the active circuits of the modem (modulator-demodulator) of the portable badge. In order to be capable of transmitting, or more precisely, in order to be capable of responding to an interrogation sent by the fixed station, the badge has a circuit in which there is at least one reflection amplifier. Depending on the biases applied, the active element, namely a field-effect transistor, behaves either as a detector for reception or as a reflection amplifier for transmission.
This type of data exchange can be applied to the control of moving bodies, for example in the identification of wagons on a railroad, automobiles at a toll-gate or pedestrians at the entrance to a building. The mobile station or badge takes the form of a chip card, and it has to be extremely economical in energy for it is supplied by small cells known as "buttons".
2. Description of the Prior Art
The general diagram of a system for the exchange of data by microwave electromagnetic radiation, according to the prior art, is fairly simple and FIG. 1 which represents it enables its elements and the abbreviations used to be specified.
A mobile station or badge 1 includes a microwave part 3, an information processing part 4 and a supply 5. The exchanges between these two parts relate to the operation of modulation/demodulation and to the transmission/reception commands.
The fixed station or beacon 2 includes a microwave source, a microwave part 6, an information processing part 7 and a computer 8 which enables the management of all the radioelectrical exchanges carried out with the badges, by means of an antenna 9 for each badge and an antenna 10 for each beacon or reader.
The following values will be used:
- d =distance between the antennas 9 and 10, PA0 - G.sub.1 =gain of the antenna 10 of the reader 2 PA0 - G.sub.b =gain of the antenna 9 of the badge 1 PA0 - P.sub.li =power transmitted at the antenna of the reader 2 PA0 - P.sub.lr =power received by the antenna of the reader PA0 - P.sub.bi =power transmitted at the antenna of the badge 1 PA0 - P.sub.br =power received by the antenna of the badge.
The modulations used in these systems are generally of the OOK (on-off keying) type in amplitude modulation or of the PSK (phase-shift keying) type in phase modulation or of the FSK (frequency shift keying) type in frequency modulation.
The working of the modem differs according to whether the beacon interrogates the badge or whether the badge responds to the beacon.
In the mode in which the badge 1 is interrogated by the reader 2, the reader 2 generates (P.sub.li) a modulated microwave signal. The badge receives this signal (P.sub.br) and demodulates it: this is a step that enables the activation of the badge that has been in the vigil state.
In the mode in which the badge 1 is responding to the reader 2, the reader generates a non-modulated microwave signal. The badge receives this signal, modulates it, i.e. loads it with the information on which it has been interrogated, and re-transmits a signal comprising, as the case may be, either losses or gain.
The electrical circuits used to perform the microwave function (modem) of the badge may be of different types.
The active circuits comprise the specific feature of giving power at the microwave frequencies of the carrier (of the order of 1 to 100 GHz). They therefore have gain at these frequencies, and use a biased transistor. These circuits are generally used during the retransmission of a signal from the badge towards the reader.
The semi-active circuits give no power at the microwave frequencies of the carrier, for which they have no gain, but nevertheless have gain at the demodulation frequencies of the order of 40 kHz to 1 MHz or several MHz, but not in the GHz range. These circuits are valuable for increasing the detection sensitivity of the badge.
There also exist passive circuits, using diodes, switches etc . . . which have no gain, whether at the microwave frequencies of the carrier (&gt;1 GHz) or at the demodulation frequencies (&lt;1 GHz). These circuits have low sensitivity for controlling the information processing circuit 4, at least in the case of a portable badge, since there is a problem of energy provided by the small "button" cells: these circuits therefore perform less well in detection.
It has been stated that the badge has only low energy sources and that it is necessary to achieve economy by limited consumption. This is why the most promising approach for communication in the badge-to-beacon direction consists in modulating the echo of a monotonous carrier wave transmitted by the beacon. It is the beacon that gives all the energy of the microwave beam, and the badge has to give only low energy, limited to the changes in the state of its circuits which cause the antenna to be matched or not matched in impedance to the frequency of the carrier (between 1 and 10 GHz). In other words, the badge modulates the echo that it sends back to the beacon, or reader.